Thermoplastic containers are well known in the art. The noted containers generally include a lid that is selectively detachable from a bowl and are commonly designed to provide a variety of features, including being reusable, disposable, microwavable, and the like.
Numerous types of bowl and lid assemblies and means for effecting sealable engagement of a lid on a bowl have been devised. Illustrative are the containers and engagement means disclosed in U.S. Pat. No. 6,170,696.
One problem associated with a conventional, microwavable container stems from the rapid temperature changes that it must endure. The container must be permitted to vent during microwaving due to the high pressures that arise as moisture in the container contents vaporizes and gas temperatures increase. Thus, the container must be left open to some degree during microwaving. However, it is also desirable to keep the bowl covered as much as possible to prevent the contents from splattering the inside of the microwave.
One common practice is to remove the lid and place it loosely over the container bowl so that air and steam can escape during microwaving. Another common practice is to only partially remove the lid (i.e., “crack open” the lid) by disengaging only part of the lid from the bowl. Both of these practices generally permit sufficient air and steam to vent during microwaving because the increased pressure within the container will tend to force the container open, increasing any space between the lid and the bowl. However, once microwaving is complete, the steam will cool and the pressure in the container will drop significantly. Because the pressure differential no longer tends to open the container, the above-noted venting practices often do not permit sufficient air to vent back into the container to compensate for the pressure drop. This problem can be aggravated by the accumulation of steam or vapor, which can form a vapor seal between the lid and bowl. A vacuum can result, and the container can be damaged and could ultimately implode.
Various container designs have been employed to provide venting. For example, in U.S. Pat. No. 3,362,565 a lid is disclosed that includes a sidewall, which has a sealing bead near the base and a shoulder near the top. The sealing bead seals against an internal shoulder provided in a container. Notches are intermittently disposed about an outer periphery of the sealing bead. These notches provide a venting passageway through which gases generated in a sealed container can escape when the gas or vapor pressure is sufficient to flex the lid upward at its center, causing a fulcruming action.
In U.S. Pat. No. 5,147,059 a lid is disclosed having a series of vent-defining protuberances on the inner surface of a vertical, outer sealing portion of a lid. The protuberances engage a container rim to permit venting when the lid is loosely placed on the container. The venting prevents internal/external pressure differentials that might resist the proper seating and removal of the lid.
There are several drawbacks and disadvantages associated with prior art container venting means. A major drawback is that the prior art venting means typically include complex design features that are difficult to manufacture. Lids employing the complex design features are thus quite costly.
It would thus be advantageous to provide a ventable spin lock container having a lid (i) that is adapted to selectively tightly seal the container in a first position and provide effective means for venting the container at a second, partially opened position and (ii) can be readily manufactured via a conventional thermoforming process.